In many applications such as electric and hybrid electric vehicles, a DC power supply is used to supply power for a three phase electric motor. Three phase motors are typically driven by a three phase power inverter. While operating the motor, AC current flows in either a positive or a negative direction in each of the three phases, with current flowing in a positive direction in one phase being returned in a negative direction in one or both of the other phases. In normal operation the sum of currents in all three phases is ideally equal to zero, and the three phases are isolated from ground. In a fault state, however, such as where there is a short between one of the phases and a motor chassis (ground), some of the current is not returned through the phases, and therefore the sum of the currents in the three phases does not equal zero.
A typical method of detecting loss of isolation in the three phases uses three current sensors to measure the current in each of the three phases. Current sensors typically detect loss of isolation by measuring each of the three phases directly with a current sensor. The three measured currents are then summed, and if the sum is higher than a predetermined threshold value (ideally equal to zero) then a loss of isolation condition is detected. This method relies on the use of a separate current sensor for each of the three phases, and relies on the accuracy of the current sensors. It can also be difficult to detect a partial short with this method.
Accordingly, it is desirable to provide an improved system and method for detecting loss of isolation in a motor. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.